NEMA updates standards for MR

New test standards for MR tackle specific absorption rate, electric fields, and geometric distortion, while a newly revised standard addresses acoustic noise. An industry executive who served on the National Electrical Manufacturers Association (NEMA) committee that developed the standards said they were created not because these issues are present problems. Rather, the standards address needs in the MR community and will be of help to equipment manufacturers and customers, as well as testing houses.

The standards can be used to set performance specifications and the criteria for acceptance testing and periodic quality assurance, according to Michael Steckner of Hitachi Medical Systems, a member of the NEMA Magnetic Resonance Technical Committee.

"All we are suggesting is that these standards can be used in a variety of ways," he said.

MS 11-2006 (Determination of Gradient-Induced Electric Fields in Diagnostic Magnetic Resonance Imaging) covers new ground for the MR industry, offering a means for measuring e-fields. These fields affect the safety and comfort of patients, Steckner explains.

"If gradients change amplitude quickly enough, people twitch," Steckner said. "That is e-fields at work, induced as a result of time-varying magnetic fields. While it's relatively easy to measure the gradient fields, it is harder to measure the e-fields - and these are really the points of interest."

MS 11 provides the ability to measure e-fields, according to Steckner.

The development of MS 12-2006 (Quantification and Mapping of Geometric Distortion for Special Applications) was motivated by the increasing use of MR in cancer radiation treatment planning. When constructing such plans, it is important to know the trajectory of the radiation beam relative to the tumor and other organs. MS 12 helps by quantifying MR geometric accuracy, Steckner said. The standard can also be helpful when planning interventional procedures, he said.

MS 10-2006 (Determination of Local Specific Absorption Rate in Diagnostic Magnetic Resonance Imaging) standardizes the measurement of local regions of SAR due to radiofrequency power deposition. It complements an earlier standard, MS 8, which addresses the measurement of whole-body average SAR. Using the two together allows complete characterization of RF power deposition, said Steckner, who noted that MS 10 provides information tables needed to construct phantoms compatible with 8T scanners.

The revised standard MS 4-2006 (Acoustic Noise Measurement Procedure for Diagnostic Magnetic Resonance Imaging Devices) better suits the gradient performance of modern MR scanners. The old standard was designed for systems whose gradients operated at frequencies lower than the first resonant frequency of the gradient, since the noise made by a gradient is generally loudest when driving at the first resonant frequency.

Therefore, to determine the MGAN (maximum gradient acoustic noise) value, the maximum sound produced by the gradients, the old standard required the gradients to pulse as quickly as possible through a specified waveform.

"The revised standard recognizes that new gradients are so fast that it is possible to drive at a frequency beyond the first resonant frequency and produce a lower (than appropriate) MGAN value," Steckner said. "By slowing the gradient waveform down, we get a more realistic measure of MGAN."